Annual Arctic ice minimum reached

Melt isn’t as bad as 2007, but still reaches number three in the record books

The verdict is in on this year’s Arctic sea-ice melt: third worst since satellites began keeping track of the northern polar cap in 1979.

Satellites and scientists continually monitor the Arctic Ocean’s skin of ice, which melts back in the summer and expands again in the winter. Researchers have been watching the ice’s decline with increasing alarm, especially after the summer of 2007 brought a record-breaking minimum. Ice extent recovered a bit in the summers of 2008 and 2009, but the long-term trend is unmistakable: The ice is shrinking in extent as well as thinning. Thinner ice is more prone to being broken up and melted away than thick ice that has already persisted for multiple years.

On September 15, the National Snow and Ice Data Center in Boulder, Colo., announced that this year’s ice seemed to have reached its minimum five days earlier, when it covered 4.76 million square kilometers. (Last year’s minimum was 5.10 million square kilometers.) At its minimum, ice has covered less than 5 million square kilometers in three of the last four years — an arbitrary cutoff to be sure, but an indicator of how the entire ice ecosystem appears to be changing. For other views of current ice cover, see here and here.

How much sea ice remains each year depends on a complex mix of factors including atmospheric patterns, ocean and air temperature and winds. This August, for instance, high pressure over the Beaufort Sea meant that ice began breaking up rapidly there compared with the previous month.

By at least one measure, this year’s minimum wasn’t that surprising. The Study of Environmental Arctic Change program runs an informal “sea ice outlook” in which people predict how much ice they think will be left at the end of the season, and why. This year the predictions made in August — 16 from scientists, two from the general public — ranged from 2.5 million to 5.6 million square kilometers. Include that extremely lowball estimate of 2.5 million kilometers, made by a member of the public, and you get a mean of … 4.8 million square kilometers, almost exactly what was seen.